Monochromatic and polychromatic ink jet printheads of the IDH type (Integrated Drive Head) are known consisting of an integrated circuit in which a plurality of thermal resistances or resistors are selectively activated by an external control circuit which transmits data or control signals to the head in order to produce the ejection of droplets of ink through nozzles located in correspondence with these resistors.
The wiring diagram of the known heads 10 (FIG. 1), both monochromatic and polychromatic, comprises, arranged in two columns, a plurality of activation groups or primitives 14 each comprising a plurality of resistors 11 and corresponding power transistors 12 (addresses I).
As is disclosed in U.S. Pat. No. 4,864,328, each primitive 14 is selected and activated by means of contacts PJ=1+M (Primitives Select) and each transistor 12 is selected and activated by means of contacts AI=1+N (Address Line Select).
Operation of the known monochromatic and polychromatic heads consists of the following steps for each column:                activating for a given time a first address I, for example the address I=1, through the contacts A1;        electrically feeding a given configuration of primitives with predetermined current pulses and through the contacts PJ=1+M;        activating in sequence a second address I;        electrically feeding a second given configuration of primitives with predetermined current pulses and through the contacts PJ=1+M;        and so on in successive steps, until activation of the N addresses I is completed.        
Whereas the wiring diagrams of the monochromatic and polychromatic printheads 10 are substantially the same, layout of the monochromatic heads 20 (FIG. 2), especially that of the activation groups or primitives 24, differs significantly in the known art from that of the polychromatic heads 30 (FIG. 3).
For example, the layout of a monochromatic head 20 (FIG. 2) of 312 nozzles comprises two columns of primitives, respectively a column of 12 odd-numbered primitives 24 and an even-numbered column, wherein each primitive 24 comprises 13 resistors 21 and corresponding nozzles:312nozzles=13*12*2wherein:                13=number of addresses I that may be selected per activation group or primitive;        12=number of primitives 24 per column; and        2=number of columns in the head.        
In particular the layout of the primitives 24 of each column, for example the odd-numbered column, comprises 12 terminals or contacts 25 suitable for being connected to an external circuit and 12 common bars 26 of resistors 21 connected to the respective terminals with linear connections, parallel to one another and uniformly distributed.
The corresponding layout of the primitives 34 (FIG. 3) of a compatible polychromatic head 30, i.e. having the same height, same number and disposition of contacts as the monochromatic head 20 (FIG. 2), comprises, for a like number of columns, 288 nozzles arranged in three colour blocks, respectively block A, B and C (FIG. 3), separated from one another by gaps D needed to physically separate the different colour blocks A, B, C.
Document U.S. Pat. No. 5,455,610 discloses a thermal ink-jet printer in which two parallel linear arrays of ejectors are subdivided into sections, the ejectors in each section being adapted to emit ink of a preselected color.
For the reasons of compatibility mentioned above, in the known art the layout of the primitives 34 of each column, for example that of the odd-numbered column, comprises 12 terminals or contacts 35 suitable for being connected to an external circuit and 12 common bars 36 of resistors 31 connected to the respective terminals.
In this case, however, according to the known art the common bars 36 are different from one another as some are connected to two adjacent colour blocks with the result that the layout of the connections between the terminals 35 and the common bars 36 is non-homogeneous, non-uniform, and of different length depending on changes to the primitive.
In addition, as the number of nozzles in the compatible polychromatic heads is less than that of the corresponding monochromatic heads, due to the gaps D, the higher order primitives are connected to a limited number of resistors.
In the example of FIG. 2:288nozzles=(13*11*2)+(13*1*2)for like addresses, primitives and columns, the higher order primitives are connected to a single resistor 31.
In short, in the known art the layout of the primitives 34 of a polychromatic head 30 is so complex and difficult to produce that in some cases the integrated circuit that constitutes the head must include a conducting layer (metal) in addition to those required for a monochromatic head in order to be able to make the interconnections.
Besides, in cases of particularly difficult layouts, the terminals corresponding to the higher order primitives of the head are not connected to the resistors with the result that one of the colour blocks, for instance the colour block C, comprises a lesser number of resistors and therefore of nozzles than the other colour blocks.
Disclosure of the Invention
The object of this invention is to produce a polychromatic printhead wherein the overall layout, particularly that of the primitives, has the same characteristics of ease and simplicity of manufacturing as in the monochromatic heads while maintaining the characteristics of compatibility, such as same number and disposition of contacts with the external circuit and same height.
This object is achieved by the polychromatic printhead wherein the connections are arranged according to a regular layout equivalent to the layout of the corresponding connections in a monochromatic printhead.
According to another characteristic of this invention, the solution identified is optimal for polychromatic printheads having a number of primitives that can be divided by the number of colour blocks envisaged.
This and other characteristics of this invention will become clear from the description that follows of a preferred embodiment and with reference to the accompanying drawings.